The present invention relates to an NMR sensor.
A measurement-while-drilling tool is described in EP-A-0581666 (Kleinberg) The tool comprises a tubular drill collar; a drill head positioned at an axial end of the drill collar; and an NMR sensor. The NMR sensor comprises a pair of tubular main magnets (which generate a static (B.sub.0) magnetic field) each located in an internal recess of the drill collar, and an RF antenna located in an external recess in the drill collar between the main magnets. The RF antenna recess is optionally filled with a magnetically soft ferrite to improve the efficiency of the antenna.
An NMR well logging system is described in U.S. Pat. No. 4629986 (Clow et al.). A pair of main magnets are separated by a gap in which a solenoid RF antenna is symmetrically disposed. The solenoid has a core of high permeability ferrimagnetic material (soft ferrite).
A problem with the prior art systems is that dimensional resonances can be induced in the ferrite by the RF electromagnetic field. This absorbs energy and reduces RF efficiency.
In accordance with the present invention there is provided an NMR sensor comprising a magnetic field generating assembly; an RF antenna; and a plurality of ferrite members which couple with RF magnetic fields transmitted or received by the RF antenna.
The ferrite members boost the Q of the RF antenna and compensate for the effects of eddy currents. Typically the ferrite members are soft ferrite members.
By splitting the ferrite into a plurality of separate members, dimensional resonance in the ferrite is minimised. In particular, this enables the ferrite members to each have a maximum dimension less than half the wavelength of the lowest order standing wave which could otherwise be set up in the ferrite.
The ferrite members may be separated by air gaps or by a suitable filler such as epoxy resin. The ferrite members may each comprise a separate particle in a single epoxy resin matrix. In a limiting case the ferrite members may be physically in contact. However at a microscopic level the members will only be in contact at points, and standing waves will still be substantially attenuated by the crystal discontinuities between the members.
Apart from minimising dimensional resonances, the provision of plural ferrite members provides an additional degree of freedom in the geometrical arrangement of the ferrite. Therefore the relative sizes and positions of the ferrite members can be selected to optimise the B.sub.0 and RF field profiles. The effect of ferrite on the B.sub.0 and RF field profiles has not previously been fully recognised in the prior art. It is important that the B field shape is optimised to maximise radial shell thickness to reduce susceptibility to lateral tool motions (such as vibration and whirl) whilst maintaining sufficient signal-to-noise ratio. In particular, unless care is taken in the design, the static magnetic field will tend to saturate the soft ferrite, reducing its relative permeability to unity and negating any improvements in RF efficiency. Similarly, the soft ferrite will modify the B.sub.0 field profile, thereby changing the shape and position of the sensitive volume from which NMR signal arises. Both of these related effects must be considered in the design of a real sensor.
Various BO field profiles are achievable by adjusting the size and axial position of the soft ferrite members: it is possible to cancel the first and second order radial gradients to create a "radially optimised" field profile, as described by Hanley in U.S. Pat. No. 5471140, or alternatively to cancel the first order axial field gradient to generate an "axially optimised" field profile, as described in EP-A-0774671, or to shim the field for uniform BO magnitude for an "intermediate" field profile, as described by Slade in PCT/GB98/02398. Unlike this prior art, the BO field manipulation is achieved using the placement of soft ferrites only; no hard ferrite permanent magnet shims need be employed.
Furthermore, in a similar fashion adjustment of the soft ferrite members can be used to reposition the small crescent-shaped resonant regions, known as "borehole lobes" and shown in FIG. 6, which can produce unwanted NMR signal from the borehole region. The lobes can be moved until they are partially or wholly within the outside diameter of the tool. In this way they cannot generate a significant borehole NMR signal.
Typically the NMR sensor is an "inside-out" sensor which performs measurements on an external sample outside the space envelope of the magnetic field generating assembly and the RF antenna.
The sensor may be employed in a variety of applications. However typically the sensor is provided in apparatus for performing borehole measurements in a formation.
The apparatus may be a wireline tool which performs measurements after the borehole has been drilled. However in a preferred example the apparatus is a measurement-while-drilling (MWD) tool which is provided with a drill head at an axial end of a support whereby the apparatus can carry out NMR measurements during drilling of the borehole. The tool may be a logging-while-drilling (LWD) or formation-evaluation-while-drilling (FEWD) tool in which the NMR information relating to the formation is stored on in-board memory for retrieval when the tool is returned to the surface. Alternatively a telemetry system may be provided and the NMR information is used to control the drill in real time (i.e. steering).
The ferrite has the unavoidable effect of reducing the inner diameter of the working volume in comparison with similar sized logging tools using permanent magnet shims as described by Hanley in U.S. Pat. No. 5471140 and EP-A-0774671. This results in a loss of penetration depth. However this is less of a disadvantage in a MWD tool because the invasion of the formation by borehole fluids occurs slowly after drilling. The MWD tool generally arrives at the formation less than an hour after cutting, whereas a wireline tool can arrive days or weeks later. As a result there will be less borehole fluid in the formation under study and so the use of ferrite is particularly suited to a MWD tool.
Furthermore a typical MWD tool has a larger radius than a comparable wireline tool. Since the B.sub.0. strength scales approximately as the second power of the magnet mean radius, it is possible to space the main magnets farther apart in an MWD tool using larger diameter main magnets and thus regain some of the penetration depth.
The ferrite members may be axially spaced and/or spaced at right angles to the axis of the tool. A primary consideration in the design of an NMR MWD tool is making the NMR measurement insensitive to the effect of lateral tool motions, such as vibration and whirl. To a first approximation it is clear that it will not be possible to re-focus the NMR signal in the sensitive region if the tool is displaced laterally (i.e. in a direction parallel to the radius) during the pulse sequence by a distance which is a significant proportion of the radial thickness of the sensitive shell. It is therefore necessary to select a B.sub.0 optimisation scheme and RF bandwidth such that the shell thickness is much larger than the maximum expected lateral displacement. Little is known about the precise motions of drilling tools down hole, but the typical range of displacement is from 1 to 10 mm at frequencies of a few Hz.
Rotation periods are between 1 and 3Hz. The typical NMR mea0surement lasts from 50 ms to 1s, so these motions are significant. However, the flexible nature of the sensor according to the present invention ensures that it is possible to design a tool with a sensitive shell thicker than the maximum expected motion. The tool described in the preferred embodiment has a shell with a radial thickness about 20 mm and axial length about 50 mm.
In comparison to a wireline borehole logging tool, an MWD tool has to be significantly stronger to support the drilling forces. In particular, as the sensor forms part of the drill collar, it has to be able to withstand the torsional and bending loads imposed by the rotating drilling action. It is therefore preferred that the entire sensor support structure is metal, such as stainless steel or titanium. However, the RF antenna will introduce localised parasitic eddy currents in the metallic structure of the tool which can seriously impair RF efficiency. It is therefore necessary to consider how to minimise the impact of the all-metal structure on the RF field.
The arrangement that gives the best mechanical strength and RF efficiency is achieved by winding the RF antenna as a solenoid in an external recess as described in EP-A-0581666. The skin depth in stainless steel at the typical operating frequency of 0.5 MHz is less than a few millimetres. Eddy currents will therefore flow in the surface of the drill collar under the RF coil, mirroring the driving current and effectively restricting the RF flux to the radial gap between the reduced drill collar outer diameter and the RF coil inner diameter. The RF coil diameter is made as large as possible consistent with the tool diameter, but it is desirable to make the coil recess as shallow as possible to minimise the loss in mechanical strength in this region. However, as the recess is made radially shallower, the gap decreases and the inductance of the RF transmit coil decreases, hence the RF field strength in the sensitive region for a fixed coil current decreases, hence requiring longer pulses, thus resulting in narrower bandwidth, reduced sensitive volume and lower signal strength. If the coil current is increased to compensate, the power requirement rises as the second power of current, so this too is undesirable. In practice the recess is made as deep as possible, consistent with adequate tool strength, and the loss in RF efficiency due to eddy currents is compensated by inserting soft ferrite into the gap between the RF coils and the recess base.
This places constraints on the design of the NMR sensor and can result in reduced mechanical strength. Therefore in a preferred embodiment the apparatus further comprises a recess formed in the support, the recess having a base and a pair of axially spaced shoulders, wherein the ferrite members are located at least partially in the recess; and one or more strengthening members which are arranged between the ferrite members, coupled to the base of the recess, and coupled to each shoulder of the recess. The strengthening member(s) increase the torsional and bending strength of the tool. As a result the depth of the recess can be greater than in the prior art without decreasing the strength of the tool.
Electromagnetic finite element analysis shows that the eddy currents flow around the strengthening member(s) and the perturbation to the RF field in the sensitive volume is minimal.
Typically the RF antenna comprises a coil which is wound over the ferrite members.
Typically the magnetic field generating assembly comprises a pair of axially spaced main magnets having opposite pole orientation (i.e. like poles facing each other), and the RF antenna is located axially between the pair of main magnets. This provides a rotationally invariant radial static magnetic field which is particularly important in a MWD tool.
Additional RF power losses will occur if a dimension of the ferrite is large enough to support a standing wave between the boundaries made by the external faces of the ferrite. The lowest mode is a half wavelength. The wavelength .lambda. is related to the RF frequency (f) and the speed of propagation of EM waves in the ferrite (v), which is in turn related to the ferrite's relative permeability (.mu.) and permittivity (.epsilon.): ##EQU1##
where c is the speed of light in vacuum. Selection of soft ferrite material with the correct combination of permeability and permittivity is therefore necessary.
Soft ferrites are all based on iron oxide compounds, but their properties are influenced by the other metallic ions in their structure. Soft ferrite used at less than 200 MHz are typically of cubic spinel crystalline structure, with chemical composition M.sup.2+ Fe.sub.2.sup.3+ 0.sub.4, where M.sup.2+ represents a metallic ion and is either Ni.sup.2+, Mn.sup.2+, Mg.sup.2+, Zn.sup.2+, Cu.sup.2+, Co.sup.2+, or mixtures of these, most commonly MnZn and NiZn. These are commonly referred to as Maganese-Zinc ferrite and Nickel-Zinc ferrite. The NiZn ferrites have typically 10000 times higher resistivity than the MnZn ferrites, so are better suited to operation above 100 kHz, due to their reduced eddy current loss. However, in addition, MnZn ferrites have electrical permittivity typically 10.sup.5, compared to about 100 for NiZn. Assuming a typical relative permeability of about 5000 for both types, the EM wave propagation velocity is therefore 1.3.times.10.sup.4 m/s in MnZn ferrite and 4 .times.10.sup.5 m/s in NiZn ferrite. So at an operating frequency of about 0.5 MHz, a half wavelength is only 13 mm in MnZn ferrite and 210 mm in NiZn. To avoid power losses due to dimensional resonances it is therefore preferred that the largest dimensions of any one piece of ferrite are less than these values. So NiZn ferrites are preferred because MnZn pieces would need to be very small and hard to make.
The choice of soft ferrite material is further complicated by the property of magnetostriction exhibited by all ferrite. This phenomenon is a microscopic change in the physical dimensions of the ferrite under the influence of magnetic field. After the application of an RF pulse, the ferrite structure "rings" like a bell as stored energy dissipates. In a practical design it is necessary to minimise the amount and duration of ringing as too much ringing can disable the NMR receiver. The most accurate NMR measurements are made when RF pulses in a CPMG sequence are applied as rapidly as possible. As the NMR echo is acquired at a point in time midway between RF pulses, it is necessary to minimise the system "deadtime"--the time taken for the receiver system to recover from an RF pulse. A high degree of magnetostriction will increase the deadtime, so it is desirable that the ferrite used has a low coefficient of magnetostriction. Unfortunately, NiZn ferrite has a coefficient of magnetostriction 3 to 5 times greater than MnZn ferrite. Therefore, if MnZn ferrite is used, the individual pieces must be small enough such that dimensional resonances are not excited--less than about 13 mm in all dimensions in the example given. Many more pieces will be required, but this can even be an advantage from a manufacturing standpoint.
To avoid resonances in the MnZn ferrite shims it is preferred to keep all their dimensions below about 13 cm. Therefore in a preferred embodiment the ferrite members are split into at least thirty seven (in this example) arc segments (as the circumference of the soft ferrite ring shims described in the 6.75" outer diameter preferred embodiment tool is 48 cm), and splitting them axially into separate axially spaced rings (at a spacing of less than 13 mm) each comprising a plurality of separate arc segments.